As shown in FIG. 1, a shadow mask of a color picture tube is generally made of a thin metal plate, and is disposed separated by about 1 cm from the phosphor face of the color picture tube in a state supported by a frame. The shadow mask is provided with about 300-350 thousand tiny holes, and electron beams emitted from an electron gun intersect the phosphor face after passing through the holes of the shadow mask. Thus, the shadow mask performs the function of separating the three basic colors so that the electron beams should be able to produce luminescence on the phosphor screen.
At amount of 15 to 20% of the electron beams emitted from the cathode intersect the phosphor screen through the holes of the shadow mask, but the residual electron beams collide with the surface of the shadow mask. As the electrical energy of the electron beams is converted into thermal energy, the shadow mask is heated to a temperature of about 80.degree. C. and that the central portion of the shadow mask, which has inferior cooling effect to the peripheral portion, is expanded toward the phosphor face, thereby inducing a doming phenomenon due to the fast progress of the thermal expansion within the color picture tube.
Due to the above described phenomenon, many of the tiny holes in the shadow mask are displaced from their original positions. This brings the result that the beams passing through the holes of the shadow mask do not arrive at the originally intended locations on the phosphor coating on the face of the color picture tube, but arrive at other locations on the phosphor coating after changing their travel paths; some land on areas which are not intended to be illuminated at the particular instant of time, and others land overlappingly on adjacent phosphors, thereby producing a thermal drift phenomenon. Consequently, the color harmony is degraded, and the colormetric purity is deteriorated, thereby making it impossible to obtain a proper color image.
Conventionally, in an attempt to overcome the above-described problem, there has been proposed a method in which, in the step of spreading an aluminum film on the inner surface of the picture tube, Mn is added, or the aluminum film is oxidized, so that the rise of the temperature of the shadow mask should be inhibited. However, this method is besieged with disadvantages, such that the process is complicated, and requires high cost. Meanwhile, there is another proposal as disclosed in U.S. Pat. No. 4,203,860 in which the amount of the backflash of the Ba getter is minimized, so that a long term thermal stabilization should be assured, as well as giving a diffraction effect to the getter material. This includes a getter material in the form of a mixed composition, but this has the disadvantage that the thermal expansion and the thermal deformation of the shadow mask can not be effectively inhibited.
In an attempt to overcome the above-described disadvantage, Japanese Patent Laid-open No. Showa 60-72143 discloses a proposal. According to this proposal, the surface of the shadow mask nearer to the electron gun is coated with a glass layer composed of an lead borate glass having a low thermal expansion coefficient, and, on the glass layer, a conductive metal compound including Ba and Al and Ni is coated as a getter film. Owing to the extremely low thermal conductivity of the glass layer, the amount of the heat transferred to the shadow mask is decreased, with the result that the thermal expansion of the shadow mask due to the temperature rise can be remarkably decreased, and that the conductive getter film can prevent the electron beams from electrical charging.
There are still other proposals, such as Japanese Patent Laid-open Nos. Showa 62-35434 and Showa 62-100934. According to these proposals, on the surface of the shadow mask nearer to the electron gun, there is spread a crystalline lead borate glass containing lead oxide (PbO) or silicon nitride (Si.sub.3 N.sub.4), in order for an electron absorbing layer to be formed. Further, on the electron absorbing layer, a conductive layer containing Ba as the principal ingredient is formed, so that the electrons temporarily charged on the surface of the electron absorbing layer should be inhibited from increasing to a higher density, and that the displacements of the courses of the electron beams should be corrected effectively by means of an electrostatic deflection, thereby effecting inhibition of the doming phenomenon.
However, in the conventional techniques described above, when forming the lead borate glass layer on the surface of the shadow mask, a high temperature heat treating facility is required, and the period of time for performing the process is extended, thereby aggravating the economy of the process.
Recently, to solve the above mentioned problems, there has been proposed a method in Japanese Patent Laid-open Nos. HEISEI 2-10626 and HEISEI 2-10627, where bismuth (Bi) material or bismuth mixed with other components is coated on the shadow mask. However, such a method has a defect that the bismuth material produces moisture (H.sub.2 O) and CO.sub.2 gas due to the temperature rising in case of firing the mixture or colliding with electron beams thereon, thereby decreasing the vacuum level within the color picture tube and deteriorating the emission characteristics of the electron beams.
Meantime, in manufacturing a color picture tube, two methods are incorporated to raise the vacuum level of the color picture tube for the improvement of emission characteristics of the electron beams, namely: an evacuating step of discharging residue air from the color picture tube using a vacuum pump to make the vacuum level about 10.sup.-6 torr, and a getter, flashing step of vaporizing getter material such as Al, Ba compound 10, and Ni compound 21 filled within a getter vessel 22 of getter 7, as shown in FIG. 2, by heating the getter material through a microwave heating means to absorb residual gas molecules, which enhance the vacuum level to about 10.sup.-7 torr.